(1) Field of Art
The present invention relates to a wheel slip control system for controlling the rotation of driving wheels so as to increase a frictional force between tires of the driving wheels and the surface of a road during acceleration of a vehicle. Particulary, it is concerned with a wheel slip control system in which the rotation of the driving wheels is controlled by the output of an internal combustion engine.
(2) Description of Related Art
Herefore, there has been known what is called an anti-skid control system for controlling the rotation of wheels so as to provide an optimum braking force without causing wheel lock during braking of a vehicle. In this type of control system, the wheel rotating speed (hereinafer referred to as "wheel speed") during braking of a vehicle is controlled somewhat lower than the vehicle running speed (hereinafter referred to as "vehicle speed") so as to provide a maximum frictional force between the wheel tires and the road surface. More particularly, as shown in FIG. 1, a slip factor S is determined by the following equation wherein Vs and V represent a vehicle speed and a wheel speed, respectively: EQU S=[(Vs-V) /Vs].times.100
When the slip factor S becomes nearly 10%, a frictional force M between tire and road surface becomes maximum and a side force F which is a drag against skid of a vehicle takes an appropriate value. For this reason, in order that the slip factor S may become nearly 10%, the wheel speed V is controlled to a value calculated according to the following equation: EQU V=(1-.alpha.) Vs-.beta.
where,
.alpha.: constant (e.g. 0.03)
.beta.: constant (e.g. 4 [km])
Thus, various proposals have heretofore been made as safety measures for controlling slip during braking of a vehicle so that the vehicle can stop at the shortest distance without skidding even when the driver applies a sudden brake. However, as to slip control during acceleration of a vehicle, it has merely been proposed to suppress the engine output upon occurrence of a wheel acceleration slip, and it has not been considered to control the accelerability of a vehicle in an optimum condition.
As a method of controlling the output of an internal combustion engine in the event of acceleration slip, it has been proposed to reduce the amount of fuel or retard the ignition timing. But, problems are involved therein such as, for example, an abrupt change of the operational condition of the internal combustion engine, causing vibrations or engine stop, and thus the control range is limited. On the other hand, in the case of controlling the amount of intake air, the output of an internal combustion engine can be controlled smoothly, providing a good drivability, but if the control is merely such that the opening of a throttle valve interlocked with an accelerator pedal is controlled against depression of the pedal, there arise problems, for example, the driver feels unpleasant or the safety cannot ensured in the event in trouble of the throttle valve controlling member. The co-pending U.S. application Ser. No. 771,774 is related Art.